Embodiments of the present invention relate generally to digitally controlled potentiometers (DCPs) and more specifically to DCPs that include a voltage-controlled resistor that compensates for resistance value errors caused by parasitics temperature, process, voltage, ageing, and other variations.
DCPs are particularly useful circuits that are found in many applications such as liquid crystal display televisions, personal digital assistants, medial devices, CD and DVD drives, set-top boxes, bar code scanners, digital cameras, and more. DCPs have three terminals, the first and second of which are the ends of a resistor string that includes a number of taps along the string, and a third node which is selectively connected to one of the taps. The selection of a tap is made using a wiper, which may be implemented using a number of transistor switches.
A DCP may have a voltage applied to the ends of the resistor string, which are the first and second nodes, and a ratioed voltage available at the third node. In this configuration, the DCP acts as a voltage divider. This configuration is useful in applications such as variable voltage regulators, analog signal processing, MP3 players, and voltage-controlled oscillators, among others.
Alternately, a DCP may also be used as a variable resistor, where the variable resistor is between the third or wiper node and one of the ends of the resistor string. This configuration is useful in applications such as programmable gain amplifiers, tunable filters, variable frequency oscillators, brightness controls for LEDs or LCDs, and industrial control applications, among others.
It is desirable to implement these various applications on integrated circuits. Unfortunately, it is very difficult to manufacture accurate DCPs on an IC. Resistors typically have variations of twenty percent or more in their value. Also, these resistances may vary with environmental changes such as temperature and power supply voltage. Further, they may be susceptible to long-term variations due to ageing.
Accordingly, what is needed are circuits, methods, and apparatus that provide DCPs having highly accurate resistance values.